GF1 Video Question

I am not now nor have I ever been a videographer. Nonetheless, my wife wants me to shoot a video of her nursery school class play tomorrow. The performance is expected to last approximately 15 minutes. The largest SD cards I have are 4GB SanDisk Extreme SDHC 30 MB/s.

I want to shoot in Motion JPEG so I can open/edit the video on my computer. What setting will assure me of being able to get the entire performance, say upwards of 20 minutes, on a single 4 GB card?